1. Field of the Invention
The present invention relates to an ink jet head which ejects an ink droplet to perform recording and a droplet ejection device having the ink jet head mounted thereon.
2. Background Art
An ink jet head is normally arranged such that a driving unit such as piezoelectric element and heating resistor is driven to pressurize an ink which has been introduced into a pressurizing chamber through an ink inlet so that the ink is ejected through an orifice. With the recent development of ink jet technique, ink jet head has not only been used in printing on paper but has also reached industrial use as constant amount droplet ejection device, including the production of wiring pattern and color filter for liquid crystal. These uses involve the use of aqueous ink as well as various liquids such as oil-based ink, solvent, strong acid and strong alkali. Therefore, the ink jet head is required to have chemical resistance. In order to meet requirements for drawing of fine pattern, the recent tendency is more ink jet heads to have a higher density for ejection of minute droplet. Thus, a technique of efficiently ejecting droplets from such a small ink chamber has been desired.
In order to eject minute droplets, a thermal ink jet system is advantageous taking into account the configuration. However, this system requires that only an aqueous ink be used as a solution to be ejected and thus cannot be put to the aforementioned industrial uses. On the other hand, a drop-on-demand piezoelectric element type ink jet head which allows deformation of a piezoelectric element to apply external pressure change to an ink chamber from outside the wall thereof so that a droplet is ejected is advantageous in that there are a wide variety of solutions which can be ejected but is disadvantageous in that pressure change can difficultly be given efficiently to the ink chamber, if it is small.
As a method for efficiently deforming the vibration plate of a small ink chamber using a piezoelectric element there has been proposed a method which comprises controlling a vibration system comprising a vibration factor of piezoelectric element and a flow path system connected to each other using a filmy piezoelectric element which undergoes deflection (see, e.g., JP-A-2003-39673).
However, the aforementioned related art technique involves the deflection of the piezoelectric element and thus is disadvantageous in that when the area of the piezoelectric element decreases with the enhancement of the density of the ink chamber, the resulting lack of deflection restricts the driving conditions for ejection of droplets and hence the range of the weight of droplet to be ejected. Referring to ink viscosity, the deflection of the piezoelectric element with respect to a high viscosity solution is inhibited because the piezoelectric element itself is not supported on a structure. In particular, when the piezoelectric element has a reduced area to meet the requirements for higher density, it is also disadvantageous in that this technique normally can difficultly perform ejection of a solution having a viscosity of 5 mPa·s or more.
On the other hand, in the case where a longitudinal vibration mode piezoelectric element is used, the vibrator takes no part in the response of the ink flow path because the piezoelectric element is mechanically connected to a structure other than the ink pressurizing chamber. Further, in the system comprising a longitudinal vibration mode piezoelectric element, the vibration plate of the ink pressurizing chamber is fixed to another structure with the longitudinal vibration mode piezoelectric element. In this arrangement, the acoustic capacity of the ink pressurizing chamber is so small that the response of the ink flow path to external input from the longitudinal vibration mode piezoelectric element is high. Moreover, since the piezoelectric element is mechanically connected to a structure, the deformation of the piezoelectric element can be efficiently transferred to a high viscosity solution as well. Accordingly, the range of viscosity of solution to which this mode can apply is wide.